The invention pertains to variable speed motors, and particularly to on line evaluation of the health of insulation in a variable speed motor.
Reliability is critical to the operation of industrial systems in which motors are employed. According to several recent studies of industrial motors, nearly 40% of all motor failures occur as the result of damage to the electrical insulation in the motor winding. Damage or excessive electrical stress to insulation may ultimately lead to catastrophic failure of the motor winding.
Insulation health is closely related to electrical discharge activity within the electrical insulation system of the motor. Electrical discharges are generally produced by differences between the potential of adjacent windings, adjacent turns, or between a winding and a metallic component such as the stator core. Turn-to-turn electrical discharge is caused by the turn-to-turn electrical stress and exacerbated by the fast rise times of new drive technology. The resulting discharge, sometimes referred to as a partial discharge event or simply discharge event, is a current spike on the order of 10-100 micro Amps, and having a duration of approximately 10-100 ns. Due to the transfer function of the motor winding and the propagation path of the discharge signal to a sensor, the discharge signal may be corrupted. This results in a broad frequency spectrum for the discharge signals generally beginning in the range of 100 kHz and extending to approximately 200 MHz or more.
Insulation systems are generally classified into either corona-resistant or non-corona-resistant. Non-corona resistant insulation systems must operate without discharge activity or premature failure will occur. Corona-resistant systems may operate for long periods with electrical discharge before failure occurs. Unless specified otherwise, all references herein are to non-corona-resistant insulation systems.
The voltage at which discharge occurs is the corona inception voltage (CIV). The higher the CIV, the stronger the winding insulation, and therefore the longer the motor life. The discharge event corresponds to a process of insulator degradation. Degradation is a complex process involving a cascade of electrons at the discharge site. This cascade accelerates in the electric field and impacts one side of the discharge site. The electrons ionize and break down the material, preferentially attacking organic species (such as the polymeric binder of the insulation system). The ions then react with available species (such as oxygen, water, etc.) and form acids. These acids then further degrade the insulation system. Furthermore, the impact of the electrons on the surface also causes localized heating, resulting in thermal degradation. Thus, measuring and monitoring the electrical discharge activity, and particularly the corona inception voltage, provides a means of assessing the health of the machine and allows one to make an estimate of risk to continued use or expected life.
For purposes of the present disclosure, motors are divided into two categories based on the type of drive signal that they employ. A first type of motor, often referred to as a synchronous motor, is driven by a simple 50 or 60 Hz sine wave. This drive signal is relatively pure, and therefore has few harmonics. As a result, the frequency spectrum of the drive signal, including harmonics, is relatively confined, and does not overlap with the signals produced by discharge events, thus, the overlapping harmonics of the drive signal can be filtered out using a very simple capacitor filter.
The second type of motor, which will be referred to herein as a variable speed motor (also known as an inverter drive motor or an adjustable speed drive motor), is driven by a pulse generator that is controlled to produce a simulated sine wave consisting of a series of square pulses. In other words, the output waveform is composed of a series of discrete pulses that are selected in their amplitude and pulse width to effect a sinusoidal waveform. While such a drive waveform powers the motor in the effectively same manner as a true sine wave, the individual square wave pulses contribute harmonics that far exceed those of a synchronous drive signal in both amplitude and breadth of spectrum. The frequency spectrum of the drive signal may exceed 5 MHz and therefore largely overlaps the partial discharge event frequency spectrum. Furthermore, since under operating conditions the drive waveform is typically on the order of 100 kV and 100 A, the drive harmonics will exceed the discharge event signal by 50 dB or more. These harmonics would saturate the typical amplifier/detection circuits commonly used for discharge detection in prior art synchronous motor applications, thereby rendering it impossible to monitor the health of the insulation system while the motor is on line. Thus the prior art methods for detecting discharge in synchronous motors are ineffective when applied to variable speed motors.
Motors may be tested in one of two modes: (1) off-line and (2) on-line. Off-line testing involves injecting a representative fast-rise time pulse(s) into the winding either at regularly increasing voltage levels until discharge activity is detected, or at a predetermined voltage level wherein a measure of the total or peak discharges are measured. In off-line testing, the motor is not performing work. On-line testing involves merely detecting the presence of discharges while the motor is operated, performing work, using the drive. If a customized drive is available, allowing variable voltage control, then inception voltage may be determined.
Accordingly, embodiments of the invention enable detection of discharge events in variable speed motors under actual operating conditions, i.e. while being driven by a variable speed drive waveform of typical operating voltage and amplitude. Further embodiments of the invention is to provide a method and an apparatus for detecting signals produced by discharge events in the presence of harmonics of a variable speed drive signal, or a fast rise time waveform. Still further embodiments of the invention is to provide a method and an apparatus for detecting the corona inception voltage for a variable speed drive motor by monitoring for discharge activity in the presence of harmonics of the variable speed drive signal or a fast-rise time waveform.
Embodiments of the invention may apply a signal obtained from the drive input of a motor to a sensor that strongly rejects high frequency drive signal components yet passes discharge signal components. The sensor thus allows discharge signal components to be detected in the presence of harmonics of the variable speed drive signal. The aforesaid sensor may be applied while varying the voltage of the drive signal and determining the lowest drive signal voltage at which a discharge event is detected, thereby identifying the motor""s corona inception voltage.
Further embodiments of the invention may determine discharge magnitude and duration relative to the input drive signal (phase-resolved analysis), and determine an electrical discharge trend that can be used to plan scheduled outage/maintenance or to provide an estimate of risk to continued operation, or that can be used for design quality assurance or manufacturing quality assurance and control.